1. Field of the Invention
The present invention relates in general to a so-called hybrid drive system for driving a motor vehicle, which drive system includes an engine and an electric motor as two drive power sources. More particularly, the invention is concerned with an improvement of such a hybrid drive system which is operable in an electricity generating drive mode wherein an electric generator or dynamo is driven by a surplus power of the engine during running of the vehicle with the engine selected as the effective drive power source, so that an electric energy produced by the electric generator is stored in an electric energy storage device.
2. Discussion of the Related Art
JP-A-5-50865 discloses an example of such hybrid drive system including two drive power sources consisting of (a) an electric motor operated by an electric energy stored in an electric energy storage device, and (b) an engine operated by combustion of a fuel. The hybrid drive system is generally adapted such that the vehicle is run with the electric motor under a relatively small load, and is run with the engine under a relatively large load. The vehicle is run in an electricity generating drive mode when a predetermined running condition is satisfied, for example, when the amount of the electric energy stored in the electric energy storage device has been reduced below a predetermined threshold. In the electricity generating drive motor, the engine is operated by combustion of a fuel so as to provide an output which is larger than a required power necessary for running the vehicle, so that a suitable electric generator is driven by surplus power of the engine. The surplus power is equal to the overall engine output minus the required power just enough to run the vehicle. In this electricity generating drive motor, the vehicle is run by operation of the engine while the electric generator is driven by the surplus power of the engine to store electric energy in the electric energy storage device. Commonly, the electric motor provided as one of the two drive power sources is adapted to also function as the electric generator. In this case, the hybrid drive system does not require an exclusive electric generator or dynamo.
In the event of some failure of the electric generator in the conventional hybrid drive system, the overall output of the engine produced in the electricity generating drive mode is used as the power to drive the motor vehicle. Since the overall output is greater than the power required to run the vehicle, the vehicle tends to be accelerated to an excessively larger extent than in the normal condition of the hybrid drive system. This excessive acceleration of the vehicle is not expected by the vehicle operator and is not desirable.